Our now-dead app got absolutely slaughtered in the ratings (IMHO rightly) simply over our use of Facebook auth. Took forever for our ratings to rebound to this[0]
Did you re-launch the app under a new name to get new ratings after removing Facebook auth? Seems like the sensible thing to do...
I put up a small visual traceroute app up on Google Play recently - https://play.google.com/store/apps/details?id=com.rc.vtrace2 - and it was selling decently (7 sales within the first few hours), but then someone put a 2 star rating and, of course, nobody has touched it since. It's pretty interesting how much impact the first few reviews actually have on an app.